As a prior art valve device of the kind specified, there is known an operating valve device which includes, as shown in FIG. 1, a main spool 5 for allowing a first hole 3 and a second hole 4, respectively to selectively communicate with an inlet port 2 formed in the valve body 1, and check valves 8 and vacuum prevention valves 9 installed between the first and second holes 3 and 4, respectively, and a first outlet port 6 and a second outlet port 7 connectable with the first and second holes 3 and 4, respectively, the arrangement being made such that when the main spool 5 is moved to the left or to the right in FIG. 1 the inlet port 2 is connected either with the first outlet port 6 or with the second outlet port 7 so as to selectively supply fluid under pressure from a pressurized fluid supply source P into either one of a first chamber B.sub.1 or a second chamber B.sub.2 of an actuator B, and also a backward flow of fluid under pressure from the first and second chambers 6, 7 into the inlet port 2 can be prevented by the check valves 8, and also fluid under pressure can be drawn by the vacuum prevention valve 9 from a tank port 10 into the first chamber B.sub.1 or the second chamber B.sub.2 thereby preventing formation of a vacuum therein or depressurization.
Stating in brief, in case the pressure in the first and second chambers B.sub.1 and B.sub.2 which results from loading F applied to the actuator B (that will be referred to as "loading pressure" hereinbelow) is higher than the pressure of fluid discharged by the pressurized fluid supply source P, there is a risk of fluid under pressure flowing from the actuator B back into the pressurized fluid supply source B, and therefore such backward flow of fluid is prevented by the check valves 8, respectively.
Further, there is a case where the actuator B is moved by a pressure loading F at a speed higher than the value corresponding to the amount of fluid under pressure supplied by the pressurized fluid supply source P. In this case, the chamber on the side of the actuator into which fluid is to be supplied will form a vacuum, and therefore the vacuum prevention valve 9 is then opened to allow the tank port 10 to communicate either with the first outlet port 6 or with the second outlet port 7 so as to draw in fluid under pressure from the fluid tank 11 thereby preventing formation of a vacuum in the chamber.
In such a prior art operating valve device, since the check valves 8 and the vacuum prevention valves 9 are installed in two rows in parallel relationship between the inlet port 2 and the first and second outlet ports 6 and 7, the distance between the inlet port 2 and the first and second outlet ports 6 and 7 becomes long so that the size of the valve body 1 will become large, thus making the whole valve device large-sized.
Further, since the vacuum prevention valves 9 are installed on the side of the outlet ports, in case the valve seat is worn away or foreign matters made ingress therein, fluid under pressure will flow from the tank port 10 into the fluid tank 11 thus increasing fluid leakage and causing such difficulties as failure in holding the actuator B in locked condition or operational delay, etc.